The US Navy relies heavily on gas turbine engines to power both aircraft and ships, spending about $2 billion (with a “b”… or about one tenth of NASA’s entire budget last year) every year on fuel for them. Even a small reduction in fuel consumption would save millions of dollars, but future engines also need to meet increasing demands for power.

Nearly all commercial aircraft—and plenty of power plants—use gas turbines, so we’ve spent a lot of time optimizing their designs. This means that trying to continue to improve them will only eke out a few more percentage points of efficiency. In order to reach both the increased power levels and reduced fuel consumption the Navy wants—10 percent and 25 percent, respectively—we’ve got to come up with entirely new engine designs. (Just like automotive engineers, as we recently featured.)

Any new technology that the Navy develops could also be used in civilian aircraft and power plants. For a little perspective, US airlines spent over $50 billion on fuel last year.

With that in mind, a team at the Naval Research Laboratory (NRL), led by Dr. Kazhikathra Kailasanath, are developing rotating detonation engines, which should offer the higher efficiency and power output desired.

Gas turbines operate on the Brayton cycle, which consists of three steps. First, a compressor raises the pressure of the incoming air. Then, fuel is injected and mixes with the air, then burns, heating everything up. Since the system is open—rather than in a closed cylinder—this process occurs at a nearly constant pressure. Finally, this hot pressurized gas expands through a nozzle to generate thrust, either for propulsion or to push a turbine and generate electricity.

The main limitation to performance is the combustion step. Like nearly every other type of burning experienced in day-to-day life, it is a deflagration, where the flame propagates at a subsonic speed.

A detonation is a flame where the reacting gases release so much energy so quickly that the burning mixture moves faster than the speed of sound, driving a shock wave. Unlike a deflagration, a detonation significantly raises the pressure, increasing the available work—without requiring any additional mechanical components. Essentially, using a detonation allows you to reach much higher efficiencies.

While concepts for engines using detonation go all the way back to Robert Goddard and even Jules Verne, practical research beginning in the 1990s focused on the pulse detonation engine (PDE) design. As the name suggests, pulsed explosions shoot out of the nozzle for thrust, 20–100 times a second. That may sound crazy, but a number of experimental PDE engines have been developed, and the US Air Force Research Laboratory even successfully flew a plane with one.

However, there are a couple of difficulties that have prevented PDEs from reaching the high efficiencies they promised. For one thing, it’s tricky to initiate a detonation repeatedly at these rates—many times a second. (You can check out our previous report on the transition from deflagration to detonation for a more detailed explanation of those challenges.)

Rotating detonation engines, or RDEs, offer a solution to that problem. Micro-injectors squirt a pressurized mixture of fuel and air into a combustion chamber the shape of a long ring (annular cylinder). Then, this gas mixture explodes, with the explosion spinning around the circumference at supersonic speeds. The high pressure produced by the detonation forces exhaust gases down and out of the chamber, where they expand through a nozzle and produce thrust that can be used to push a turbine or aircraft.

Unlike a PDE, which requires repeatedly initiating detonations, an RDE only requires one detonation that spins around and around the chamber, continuously producing thrust.

Of course, there are a number of challenges with this design as well. The materials must be able to withstand the high pressure and temperature from the detonation. Also, the detonation occurs right near the injector inlets, where the strong pressure could actually push gases backwards.

To further complicate matters, since this is a relatively new concept, we don’t really understand (yet) the forces and heat fluxes that the combustion chamber experiences, making it difficult to optimize the design. By conducting simulations of RDEs with various fuels, researchers at NRL are trying to better understand the flow physics inside the combustion chamber. In a recent paper, simulation results showed that RDEs fueled with different hydrocarbons could reach fuel efficiencies, measured in specific impulse, of 85 to 89 percent of an ideal detonation cycle.

Promoted Comments

Why bother with gas turbines for ship propulsion when nuclear exists? Nuclear powered ships are refueled about once every 25 years.

Because nuclear reactors are incredibly expensive to build and operate, and the Navy has a lot of smaller ships that don't have the manpower or need. Right now the only viable ship types for nuclear are carriers, because they are big enough that the efficiency and available max power actually matters, and subs, where the lack of emissions and silence outweigh the downsides. A nuclear reactor also mean that a ship will have to maintain a much higher level of security for its entire life, and has to be completely decommissioned instead of re-purposed or sold off when the Navy is done with it.

12829 posts | registered Aug 6, 1999

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Apollo: The Greatest Leap

Apollo: The Greatest Leap

In honor of the 50th anniversary of the beginning of the Apollo Program, Ars Technica brings you an in depth look at the Apollo missions through the eyes of the participants.

Kyle Niemeyer
Kyle is a science writer for Ars Technica. He is a postdoctoral scholar at Oregon State University and has a Ph.D. in mechanical engineering from Case Western Reserve University. Kyle's research focuses on combustion modeling. Emailkyleniemeyer.ars@gmail.com//Twitter@kyle_niemeyer

I'm wondering how shock waves and detonations are for marine wildlife? Efficiency is good, except when it comes at a cost to the environment.

I'm not sure what you mean - if you're picturing massive shockwaves pulsing in the sky, that isn't really the case. This wouldn't be any worse for the environment than a conventional turbofan, and with its added efficiency it's indirectly better for the environment due to lower fuel consumption.

Edit: Thought this was about airplanes. I'm an idiot. This would be even less of an issue in a ship though, as the engine is deep inside it (rather than external, like a plane).

I wonder if using different hydrocarbons would present more trouble than it's worth from a fuel-supply logistics standpoint. If you're low on fuel, everyone else around you burning something else could make refueling a tricky proposition.

I guess I envisioned a huge engine, similar to a jet engine. Everyone knows how loud those are. So I take it this thing runs quiet then? How big of an engine are we talking about here? It still seems to me that a shockwave could travel rather easily through the hull of the ship and into the water. How is the shockwave contained anyway? Forgive me, I am curious.

Edit- I apologize for the earlier question too, I didn't realize my curiosity was frowned upon.

I wonder if using different hydrocarbons would present more trouble than it's worth from a fuel-supply logistics standpoint. If you're low on fuel, everyone else around you burning something else could make refueling a tricky proposition.

You have to start somewhere, and eventually it wouldn't be just one ship that is different. If this works as well as they hope that it will, I am guessing there would be a gradual phase in/out of a LOT of ships, and a great deal of airplanes. It may take a very long time to totally replace them though. Some of our airframes that we are flying today are 50 years old, and going strong.

Having spent 5 years working in a machine shop that made fuel injectors for gas turbines, military and civilan, I can say that a big part of the problem in increasing efficiency in them is that the injectors, especially for the afterburners, just melt away under the extreme heat. We would make these beautiful injectors that simply disintegrated after 2 or 3 uses making any attempts at efficiency a joke because they went from precision sprayers to basically a fire hose.

It stands to reason that a new engine tech is needed as GT's aren't going to cut it.

I wonder if using different hydrocarbons would present more trouble than it's worth from a fuel-supply logistics standpoint. If you're low on fuel, everyone else around you burning something else could make refueling a tricky proposition.

Part of it probably would come down to if the Navy's existing tankers could easily carry both types of fuel. The other major question would be if the existing turbines and new RDE engines could burn the others fuel without damage or significant loss of power.

I'm wondering how shock waves and detonations are for marine wildlife? Efficiency is good, except when it comes at a cost to the environment.

I suppose it is possible if there is more "thrum" to these engines that the additional sound could be disruptive. But I doubt it's much different than current sounds.

Still, I'm hopeful that as these technologies are developed we will see concern for the waves as well as the ships.

To this: Edit- I apologize for the earlier question too, I didn't realize my curiosity was frowned upon.

I would imagine that posting what looks very much like an over-zealous "greenpeace, think of the planet," type of post in an article that will mostly get military and science type replies is the reason you're getting down-voted. Combining both your posts would have looked more in-line with expected posts.

The US Navy relies heavily on gas turbine engines to power both aircraft and ships, spending about $2 billion (with a “b”… or about one tenth of NASA’s entire budget last year)

Ugh. Why use this comparison? Needless flame-baiting.

edit: english verbologyedit2: wow, me type no good.

Only a racist would say "race-baiting".

They are using analogies so that the public can better understand the gravity of the issue. Maybe for many people, what the military spends on fuel means nothing for them. So, providing analogies and context that people can understand might put pressure on Congress to support the military's research into "alternative fuel and efficiency" that they sorely need, especially given the budget constraints in today's environment.

I wonder if using different hydrocarbons would present more trouble than it's worth from a fuel-supply logistics standpoint. If you're low on fuel, everyone else around you burning something else could make refueling a tricky proposition.

Sorry if that part was confusing—I didn't mean that an actual engine would use multiple hydrocarbons as fuels, just that they were testing various fuels. Ideally, these would eventually use standard jet fuels just like gas turbine engines.

I guess I envisioned a huge engine, similar to a jet engine. Everyone knows how loud those are. So I take it this thing runs quiet then? How big of an engine are we talking about here? It still seems to me that a shockwave could travel rather easily through the hull of the ship and into the water. How is the shockwave contained anyway? Forgive me, I am curious.

Edit- I apologize for the earlier question too, I didn't realize my curiosity was frowned upon.

i think you are getting too carried away with the description. the combustion engines we have now can have equally scary descriptions with the explosions they create to move pistons.

at the end of the day all its doing is moving the propeller of the ship not creating a wake of explosions behind it to move.

If they can manage to get Bussard's fusion generator working, then great, but there's no indication it will be any closer to being viable in a few years than it is now. Even if they are able to prove that it's a viable way forward, it will probably be a decade or more before we see it in any kind of in-the-field prototype for a ship. I would say the whole think stinks of scam, but the Navy seems to believe it still has promise, so I'll trust their judgement.

The US Navy relies heavily on gas turbine engines to power both aircraft and ships, spending about $2 billion (with a “b”… or about one tenth of NASA’s entire budget last year)

Ugh. Why use this comparison? Needless flame-baiting.

edit: english verbologyedit2: wow, me type no good.

Only a racist would say "race-baiting".

They are using analogies so that the public can better understand the gravity of the issue. Maybe for many people, what the military spends on fuel means nothing for them. So, providing analogies and context that people can understand might put pressure on Congress to support the military's research into "alternative fuel and efficiency" that they sorely need, especially given the budget constraints in today's environment.

You're saying I am inviting the flames myself by responding to that pointless aside I quoted? Fair enough, but your choice of analogies is offensive, I might say.

Well,this certainly seems like a good idea now, but if I'm honest,it's not entirely sound in the long run as fuel prices would continue to creep forward as the supply of easier to extract oil dwindle. I know it might sound silly,but a simpler choice would be to consider going back to nuclear power for major surface combatants(let's say DDGs and up) - perhaps even save from RnD and adapt a submarine's reactor for surface needs, this way there propulsion will be oil independent and will have better endurance than a COGAG-powered ship. A side benefit is that the USN already has the support infrastructure and know-how to maintain nuclear-powered vessels, so it won't be a too traumatic changeover..

Also,obligatory XKCD link regarding the whole

Quote:

Hopefully, in a few years, this progress will lead to a working engine.

I guess I envisioned a huge engine, similar to a jet engine. Everyone knows how loud those are. So I take it this thing runs quiet then? How big of an engine are we talking about here? It still seems to me that a shockwave could travel rather easily through the hull of the ship and into the water. How is the shockwave contained anyway? Forgive me, I am curious.

Actually, there is only one detonation. All it really is is an alternative to the combustor of a normal gas turbine engine. The hot gases created by the detonation expand through the turbine, transferring energy to it and becoming colder, then exhausting out the top of the exhaust stack as normal. The shockwave has to stay in the detonation chamber in order to keep detonation going. In principle, from the outside, what you would hear is no different than the sound a normal gas turbine would make.

Quote:

Edit- I apologize for the earlier question too, I didn't realize my curiosity was frowned upon.

The problem was that you jumped to a conclusion from an erroneous assumption that was already addressed in the article. Don't worry though, I voted this post up!

This is what some of those supercomputers are used for. Three dimensional simulations take a whale of a lot of computing power. Hopefully, the new generation will allow much better simulations.

You're absolutely right—finding way to do those simulations is actually my area of research! I didn't mention it in the article, but they've treated the chemistry in a simple way so far, using a single "reactant" species to represent the fuel and air and a single "product" species to represent all the products. Instead of actually calculating all the reactions that occur and intermediate species, they just assumed a single, perfect reaction step and looked up the resulting temperature and pressure.

To a certain extent, this assumption is ok since the reaction zone in a detonation is so small, but they'll need to use more accurate and detailed (and therefore more computationally expensive) models to get more realistic results.

I'm wondering how shock waves and detonations are for marine wildlife? Efficiency is good, except when it comes at a cost to the environment.

The detonations are about as dangerous as the combustions you get in today's combustion engines. Your cars engine doesn't kill nearby wildlife because all the "setting stuff on fire and blowing it up" business happens inside the engine, not outside it.

And the same would be the case here. The entire point is to drive a turbine, and you can't do that by blowing up the countryside. You have to direct the force of the detonation towards your turbine, and not towards the dolphins swimming near your ship. Otherwise it's just useless.

Why bother with gas turbines for ship propulsion when nuclear exists? Nuclear powered ships are refueled about once every 25 years.

Maybe you missed the part of the article which mentioned that this would also be useful outside the navy? Or to the airline industry (A nuclear reactor on every airplane!)? Or maybe you believe that there's no point in trying to research new engine designs at all, ever, because we've got nuclear?

Or maybe you're forgetting that uranium is also a limited resource? One which, some estimate, is going to start running out in 50 years or so at the current rate of consumption. (Of course there's a lot of uncertainty on that one)

Why bother with gas turbines for ship propulsion when nuclear exists? Nuclear powered ships are refueled about once every 25 years.

Because nuclear reactors are incredibly expensive to build and operate, and the Navy has a lot of smaller ships that don't have the manpower or need. Right now the only viable ship types for nuclear are carriers, because they are big enough that the efficiency and available max power actually matters, and subs, where the lack of emissions and silence outweigh the downsides. A nuclear reactor also mean that a ship will have to maintain a much higher level of security for its entire life, and has to be completely decommissioned instead of re-purposed or sold off when the Navy is done with it.

The US Navy relies heavily on gas turbine engines to power both aircraft and ships, spending about $2 billion (with a “b”… or about one tenth of NASA’s entire budget last year)

Ugh. Why does this relevant?

Because Navy=Evil, while NASA=Good./s

@harteman - I suspect the downvotes were because you mentioned the environment.re: detonation: these aren't big, impressive-looking BOOOOOOOOOM explosions. They're closely akin to the "bang" in your car engine - from the article, 20-100 times per second (1,200-6,000 "RPM," to carry the analogy.)

The shockwave is used to produce work within the system/"engine," and by the time it gets to the ocean, it's no more "shockwave" than the vibration of an old diesel engine.

Cavitation from the propellers is likely more hostile to sea life than the rotating detonation engines.[disclaimer - I am not a combustion engineer)

Having spent 5 years working in a machine shop that made fuel injectors for gas turbines, military and civilan, I can say that a big part of the problem in increasing efficiency in them is that the injectors, especially for the afterburners, just melt away under the extreme heat. We would make these beautiful injectors that simply disintegrated after 2 or 3 uses making any attempts at efficiency a joke because they went from precision sprayers to basically a fire hose.

It stands to reason that a new engine tech is needed as GT's aren't going to cut it.

I'm wondering if the advances in growing diamonds would make it possible to use diamond in cases like this. They have been boasting that we are entering the age of diamond. They will be used in chip manufacturing to replace silicon thanks to diamond's extreme high heat tolerance.

Or maybe you're forgetting that uranium is also a limited resource? One which, some estimate, is going to start running out in 50 years or so at the current rate of consumption. (Of course there's a lot of uncertainty on that one)

More than a lot of uncertainty, not even including reprocessing and use of mixed-oxides.

@harteman - I suspect the downvotes were because you mentioned the environment.re: detonation: these aren't big, impressive-looking BOOOOOOOOOM explosions. They're closely akin to the "bang" in your car engine - from the article, 20-100 times per second (1,200-6,000 "RPM," to carry the analogy.)

Actually, if you're hearing a "bang" in your engine that is a detonation, that would be knock and it's quite harmful. The normal combustion in a car engine is deflagration, which is subsonic. The 20–100 times a second in the article was referring to pulse detonation engines, not a car engine.

Back in the day I worked on the control systems for gas turbines on Navy ships. It was my understanding that strategy for propulsion was to be electric drive.

Currently ships can have at-least 7 gas turbine engines, 4 big ass engines for propulsion (LM2500s) that need to be low in the ship (below the water line) and aligned with the reduction gears and screws. The other 3 smaller engines were for electric generation (TF-40B.)

The plan was direct drive electric screws, and maybe 3 of the same type of engine that could be placed anywhere in the ship. It would greatly reduce the complexity and the amount of equipment needed to drive the ship. It was to reduce crew size, make ships more survivable in an attack, and provide power for electrical weapon systems (rail guns)

I guess I envisioned a huge engine, similar to a jet engine. Everyone knows how loud those are. So I take it this thing runs quiet then? How big of an engine are we talking about here? It still seems to me that a shockwave could travel rather easily through the hull of the ship and into the water. How is the shockwave contained anyway? Forgive me, I am curious.

Edit- I apologize for the earlier question too, I didn't realize my curiosity was frowned upon.

Currently ships have about 7 jet engines in them. The same engines that power the C-5 Galaxy power modern destroyers. (GE LM2500s)

Jet engines are noisy because the outlet directly to the atmosphere. The stacks on ships act as big mufflers, and make them much nice to be around.

Having spent 5 years working in a machine shop that made fuel injectors for gas turbines, military and civilan, I can say that a big part of the problem in increasing efficiency in them is that the injectors, especially for the afterburners, just melt away under the extreme heat. We would make these beautiful injectors that simply disintegrated after 2 or 3 uses making any attempts at efficiency a joke because they went from precision sprayers to basically a fire hose.

It stands to reason that a new engine tech is needed as GT's aren't going to cut it.

Good thing ships dont have afterburners. I used to inspect ship engines. Most of the problems were either maintenance induced failure, or post combustion. The injectors only get damaged when the combustion chamber is messed up and the flame front gets too close to the injector.

(Nuclear) subs, where the lack of emissions and silence outweigh the downsides.

I don't think nuclear subs are quieter than diesel-electrics because the reactor requires all sorts of support equipment, like pumps, that have moving parts and require a vibration damping to cut the noise. Electric batteries are far superior from a noise perspective as there is an absolute minimum number of moving parts.

(Nuclear) subs, where the lack of emissions and silence outweigh the downsides.

I don't think nuclear subs are quieter than diesel-electrics because the reactor requires all sorts of support equipment, like pumps, that have moving parts and require a vibration damping to cut the noise. Electric batteries are far superior from a noise perspective as there is an absolute minimum number of moving parts.

Yeah, nuclear subs are really more about endurance. Having said that, modern U.S. nuclear subs are about as quiet as diesels (when the diesels are on battery, that is), but that's the result of tons of R&D money, not because nuclear is inherently quieter.

I'm wondering how shock waves and detonations are for marine wildlife? Efficiency is good, except when it comes at a cost to the environment.

The Navy isn't going to invest in an engine that lets shockwaves slip out into the ocean. That would light their vessels up like a firework on enemy sub radars. I would assume the engine would absorb/utilize the energy efficiently, and if any minor amount slipped by they'd have some kind of dampening system to limit their sound profile in the ocean.

Diesel subs are louder when you turn them on, quieter when you turn them off. Range is reduced compared to nuclear.

You can't turn a nuclear reactor on and off willy nilly, so nuclear subs tend to be "louder" than a diesel-electric running on batteries with the engine shut off.

Differences in acoustic isolation and other technological discrepancies make the issue trickier, but if you want your submaries to go all around the globe, instead of sitting at home and waiting, and you want them to be as stealthy as possible throughout that whole process, nuclear is the way to go.